1. Field of the Invention
The present invention relates to a multi-mode communication terminal device capable of communicating in various communication modes corresponding to a plurality of communication systems.
2. Description of the Related Art
FIG. 5 is a flowchart showing the conventional signal receiving process performed in the automotive-use telephone systems currently adopted in the North American countries, and more particularly showing the signal receiving process conducted in a communication terminal device supporting three different modulation modes within the same frequency band, namely the FM modulation mode, the xcfx80/4 DQPSK mode and the GMSK mode, which are adopted in the communication systems such as the AMPS (Advanced Mobile Phone System), the North-American TDMA (Time Division Multiple Access) system, and the CDPD (Cellular Digital Packet Data) system, respectively. The signal receiving process here includes the process for identifying the modulation mode of these communication systems and controlling the calling process thereof.
Next, the procedure for carrying out this signal receiving process is now explained.
First of all, in step S30, hardware of the communication terminal device is set to comply with the AMPS system, namely the FM modulation mode, and thereafter the scanning operation with respect to the control channels is performed by the RSSI (Receive Signal Strength Indicator) in step S31.
Then, in step S32, the control channels whose RSSI value is detected to be greater than the predetermined value are captured and the bit demodulation of the thus captured channels is carried out in step S32, which is followed by the synchronizing process thereof in step S33 in accordance with the procedure for receiving FM-modulated signals. At this stage, if the synchronization in step S33 is completed successfully, the procedure then advances to step S34 in which the calling process of the AMPS system is carried out. On the other hand, if the synchronization is unsuccessful, the procedure advances to step S40, in which hardware of the communication terminal device is set to comply with the North-American TDMA System, namely the xcfx80/4 DQPSK mode, and thereafter the RSSI scanning operation with respect to the concerned channels is performed in step S41.
After these operations, in step S42, the channels whose RSSI value is detected to be greater than the predetermined value are captured and the bit demodulation of the thus captured channels is carried out in step S42, which is followed by the synchronizing operation thereof in step S43 in accordance with the procedure for receiving the xcfx80/4 DQPSK-modulated signals. At this stage, if the synchronization in step S43 is completed successfully, the procedure then advances to step S44 in which the calling process of the North American TDMA system is carried out. On the other hand, if the synchronization is unsuccessful, the procedure advance to step S50, in which hardware of the communication terminal device is set to comply with the CPDP system, namely the GMSK mode, and thereafter the RSSI scanning operation with respect to the concerned channels is again carried out in step S51.
After these operations, in step S52, the channels whose RSSI value is detected to be greater than the predetermined value are captured and the bit demodulation of the thus captured channels are carried out in step S52, which is followed by the synchronizing operation thereof in step S53 in accordance with the procedure for receiving the GMSK-modulated signals. At this stage, if the synchronization in step S53 is completed successfully, the procedure then advances to step S54 in which the calling process of the CDPD system (or registration in this case) is carried out.
On the other hand, if the synchronization is unsuccessful, the procedure goes back to step S30, in which hardware of the communication terminal device is set to comply with the AMPS system, and the same procedures starting therefrom are repeated again.
In the aforementioned procedures, however, the RSSI scanning operation is carried out with respect to only some or all of the predetermined channels in the communication mode of each communication system. In other words, since the signal receiving procedure is conducted in such a manner that the RSSI scanning operation is carried out with respect to a large number of channels and thereafter those channels whose RSSI value is detected to be greater than the predetermined level are captured, this procedure may require a substantial time in some cases.
Further, in accordance with the aforementioned procedures, since the scanning operation for signals of even unused communication systems is carried out in the regions where only one system is employed, it may produce a waste of time, and the total power consumption may thereby be increased.
Still further, since even the channels captured in accordance with the result of the RSSI scanning operation whose RSSI level is greater than the predetermined RSSI level may not be able to receive the same signal in other different communication systems, the user of the communication terminal device will lose the best timing for receiving the signal.
Since the conventional terminal device is configured as mentioned above, there has been such a problem that in a case in which the terminal device corresponds to many communication systems, the time required for identifying the signal modulation mode of each communication system and carrying out the signal receiving process will be greatly increased, and thus the total power consumption may also be increased.
The present invention has been proposed to solve the problems aforementioned, and it is a first object of the present invention to provide a communication terminal device which is capable of identifying the received signal in a signal modulation mode decision circuit (hereinafter may be referred to just as a xe2x80x9cdecision circuitxe2x80x9d), reducing the time required for deciding the signal modulation mode, and also capable of reducing the total power consumption.
The second object of the present invention is to provide a communication terminal device which is capable of identifying the received signal in a signal modulation mode decision circuit, reducing the time required for deciding the modulation mode, and also capable of reducing the total power consumption, as well as minimizing the overall size of the decision circuit.
The third object of the present invention is to provide a communication terminal device which is capable of reducing the time required for deciding the signal modulation mode and the total power consumption, and also capable of selecting a desired communication system out of a plurality of different communication systems, when many receivable signals of different communication systems are received.
In order to achieve the above objects, the communication terminal device according to the first aspect of the present invention is configured in such a manner that it comprises: a plurality of demodulation circuits that correspond to the received signals of the same frequency band, which signals being modulated in the respectively different modulation modes, a decision circuit for determining the modulation mode of each of the received signals whose reception level is greater than a predetermined level, on the basis of the frequency component of the received signal, and a control circuit, which puts one of the demodulation circuits that corresponds to the modulation mode of the received signal to an active state on the basis of the decision made by the decision circuit, and simultaneously puts the decision circuit to an inactive state.
In this construction, the control circuit puts all other demodulation circuits than the activated demodulation circuit to an inactive state, as well as the decision circuit.
The communication terminal device according to another aspect of the present invention is configured in such a manner that a first demodulation circuit among the plurality of demodulation circuits is an FM demodulation circuit, which first demodulation circuit being composed of a front-stage section and a rear-stage section, and the decision circuit inputs the frequency component of the received signal from the front-stage section that comprises therein a signal strength indicator and a discriminator, wherein a second demodulation circuit in the plurality of demodulation circuits is a GMSK demodulation circuit, which GMSK circuit sharing the front-stage section of the first demodulation circuit.
The communication terminal device according to further aspect of the present invention is configured in such a manner that the decision circuit further comprises: a first band-pass filter that allows only a first frequency component fed from the signal strength indicator, a second band-pass filter that allows only a second frequency component fed from the discriminator; a first integrator that integrates the first frequency component fed from the first band-pass filter and identifies its signal modulation mode; a second integrator that integrates the second frequency component fed from the second band-pass filter and identifies its signal modulation mode; and a signal modulation mode judging section that receives the outputs from the first and second integrators, and discriminates at least three signal modulation modes of the received signals.
In this construction, if there are a plurality of signal modulation modes determined at the decision circuit, the control circuit applies a predetermined priority order to the plurality of modulation modes.